Flush the containers with a gallon or two of RO water..you seem to have some nute build-up that is causing the lower PH runoff, if you were to test the PPM of that runoff I can guarantee it's signifigantly higher than whats going in...you should regularly be flushing anyways..say every 10-14 days or so to prevent any salt build-up. But note, when flushing it's important to follow-up with a balanced nute solution as the plants will actually start to wilt and can dehydrate because of the changes in the osmotic pressure of the root system due to the zero ppm flush and the leaching this causes from the plant itself potentially...so make sure you always follow with fresh balanced nutes so no harm is done.